A progressive roll is the basis for all Skill rolls. A normal Skill roll is 1d6 + Skill. Each point that this skill exceeds the target number (such as they Body attribute + Armor for a physical attack) generates one success.

When an adventurer rolls a 6 on a Skill roll they count that roll as 5 and roll again, adding the new roll to the total. Each time they roll a 6 they again add 5 and roll again. A progressive score can grow quite large - the weakest attacker using a lowly dagger could roll 50 or more successes and bring down the largest dragon.

I really like that progressive rolls account for the real-world variation we call "luck." Is this common in RPGs?

Progressive rolls exit in a number of games. The most famous is Shadowrun by FASA. The idea in Shadowrun was that the game only used a d6, which provided only a limited ability to simulate real world events. By making a d6 roll progressive it allowed for lucky rolls that could, in the extreme, cause a Hada to kill a dragon with a soup spoon. This may be an unlikely outcome, but it is a possible outcome. Winning the lottery is an unlikely outcome, but it is a possible outcome that happens every couple of weeks to one out of millions on millions of player.

Shadowrun's failure of course was that the statistical system was out of whack and unbalanced. However, most people quit trying to play the game strictly by the rules and instead simply let the dice indicate where the action was going.